Device and method for monitoring and controlling physiologic parameters of a dialysis patient using segmental bioimpedence

ABSTRACT

The present invention includes a method of determining the dry body weight of a patient undergoing dialysis by means of segmental bioimpedance analysis. In preferred embodiments, dry body weight is determined by comparison to the bioimpedance values of normal subjects or by monitoring changes in bioimpedance during dialysis. One embodiment of the present invention is a device for determining dry body weight during dialysis.

FIELD OF THE INVENTION

The present invention relates to a device and method that utilizesegmental bioimpedance for monitoring and controlling physiologicparameters of a dialysis patient.

BACKGROUND OF THE INVENTION

Accurate assessment of a dialysis patient's hydration status andprediction of dry body weight (DW or dry weight) is a major problem inthe clinical management of the dialysis patient. In both hemodialysisand peritoneal dialysis patients, dry weight is the target weight at theend of dialysis treatment which best reflects removal of excess waterfrom the body. In clinical practice, estimation of DW is an impreciseundertaking, and depends to a large extent on the treating physician'sinterpretation, based on his or her medical experience and familiaritywith the particular patient's condition, of clinical symptoms and signssuch as changes in the blood pressure, pulse, and weight of the patient.The correct interpretation of such signs and symptoms is complicated bythe fact that the pre-treatment body weight varies for each treatment,the amount of excess fluid is not constant and the amount of fluid thatcan or should be removed from any particular patient during anyparticular dialysis treatment may be limited by an individual'scardiovascular tolerance, often manifested by clinical signs andsymptoms, such as pretibial edema, dyspnea, cramps and/or a decline inblood pressure. Alternatively, an overestimation of the amount of fluidto be removed may result in potentially avoidable symptoms,unnecessarily lengthy dialysis treatments and often prolonged stays atthe dialysis facility. Therefore, over- or underestimation of DW willsignificantly affect both the efficiency of dialysis treatment andpatients' quality of life.

Bioelectrical impedance analysis (BIA) has been recognized as anoninvasive and simple technique to measure body hydration and hydrationstatus (i.e. over-, under- or normal hydration) of subjects for morethan twenty years. There is substantial literature on using BIA for thestudy of dry weight. Kouw et al proposed a method to measure changes inregional conductivity, and then to measure regional extracellular volume(ECV) and intracellular volume (ICV) by BIA. See, P. M. Kouw, et al,Assessment of post-dialysis dry weight: an application of theconductivity measurement method. Kidney Int. 41:440-444,1992. However,Kouw's method cannot be used to measure interstitial fluid alone as itdoes not distinguish between interstitial fluid and plasma, both ofwhich make up the ECV compartment. Piccoli published a method of BIAvector analysis which uses the ratio of resistance to reactance toidentify dry weight. While this technique could be used to compare thesubjects' body hydration, it is unable to predict individual patient'sdry weight because of the significant variation in measured values. See,Piccoli A: Identification of operational clues to dry weightprescription in hemodialysis using bioimpedance vector analysis. KidneyInt. 5 3:1036-1043,1998.

Recently, there have been increased numbers of dry weight studies usingblood volume (BV) measurements. See, for example, J. P. de Vries et al,Non-invasive monitoring of blood volume during hemodialysis: Itsrelation with post-dialytic dry weight. Kidney Int 44:851-854,1993, andJ. K. Leypold, et al, Determination of circulating blood volume bycontinuously monitoring hematocrit during hemodialysis. J. Am. Soc.Nephrol. 6:214-219,1995. Blood volume measurement is a noninvasivetechnique that can be used to indicate water concentration in blood,i.e. hematocrit, during hemodialysis, but it cannot be used to directlydetermine dry weight because changes in blood volume are mainlydependent on the rate of vascular refilling which, in part, isindependent of body hydration. See, e.g., J. K. Leypoldt, et al,Evaluating volume status in hemodialysis patients. Adv. Ren. Replace.Ther. 5:64-74,1998. On the other hand, since a change in the hematocritlevel may alter conductivity in the blood during dialysis, it isdifficult to obtain information about tissue hydration by eithertraditional bioelectrical impedance analysis or blood volume analysis.To date, a major problem has been how to measure resistivity of bloodand tissue separately, in order to estimate the fluid volume in theintravascular compartment and the interstitial compartment,respectively.

Thus, there is a need for a precise, easily used and operatorindependent method for determining the hydration status of a dialysispatient, identifying or predicting the dry weight of such a patient andcalculating the amount of fluid that should be removed during a dialysissession. In addition, there is a need for a method of controllingdialysis in response to a patient's hydration status.

SUMMARY OF THE INVENTION

The present invention includes a method for determining the hydrationstatus of a dialysis patient comprising the steps of measuring theresistivity of a body segment of the patient, correlating the measuredresistivity with predetermined normal dry weight values, and derivingthe patient's hydration status. Optionally the resistivity of theinterstitial fluid in the body segment is measured to derive thepatient's hydration status. In one embodiment, the resistivity of thebody segment is determined while applying a pressure of at least aboutsystolic blood pressure, optionally from about 120 mmHg to about 240mmHg. The body segment can be a limb segment, preferably a thighsegment.

Included, is a method for determining a hemodialysis patient's dryweight comprising the steps of periodically measuring the resistivity ofa body segment during hemodialysis; comparing successive resistivitymeasurements; and identifying the patient's dry weight when asubstantially constant resistivity is reached. Optionally, resistivityis measured from about every 5 minutes to about every 20 minutes duringhemodialysis, preferably about every 10 minutes during hemodialysis. Inone embodiment the resistivity of the body segment is measured at apressure of at least about systolic blood pressure, optionally fromabout 120 mmHg to about 240 mmHg.

The present invention includes a method for dialysing a patient to thepatient's dry weight that comprises measuring the resistivity of a bodysegment of the patient, correlating the measured resistivity withpredetermined normal dry weight values, deriving the patient'shydration, and continuing hemodialysis until the resistivity of the bodysegment correlates with the predetermined normal dry weight values,preferably measuring the resistivity of the body segment at a pressureof at least about systolic blood pressure.

Also provided is a method for hemodialysing a patient to the patient'sdry weight comprising the steps of periodically measuring theresistivity of a body segment during hemodialysis, comparing successiveresistivity measurements, and discontinuing hemodialysis when asubstantially constant resistivity is reflected. Preferably, theresistivity of the body segment is measured at a pressure of at leastabout systolic blood pressure. In this embodiment, the resistivity ofthe body segment is measured from about every 5 minutes to about every20 minutes during hemodialysis.

The present invention also provides a method of monitoring the heartrate of a hemodialysis patient comprising the steps of determining atime interval between two successive bioimpedance wave peaks andmultiplying the reciprocal of the time interval by 60 to obtain theheart rate, and a method of calculating the cardiac output of a patientin need thereof comprising the steps of measuring the stroke volume inan arm segment by bioimpedance analysis, substantially simultaneouslymeasuring the stroke volume in an ipsalateral leg segment bybioimpedance analysis, summing the stroke volume in the arm segment andthe stroke volume in the leg segment, and multiplying the sum by twicethe heart rate to obtain the cardiac output. Preferably, the strokevolume of the arm segment is calculated by applying an external maximumpressure to the arm segment and determining the change in blood volumein the arm segment between the point of maximum pressure and the pointat which no external pressure is applied divided by the number of heartbeats between the two points in time, and the stroke volume of the legis calculated by applying an external maximum pressure to the legsegment and determining the change in blood volume in the leg segmentbetween the point of maximum pressure and the point at which no externalpressure is applied divided by the number of heart beats between twopoints in time.

Included is a device for controlling a hemodialysis machine comprising abioimpedance analysis measurement unit in electrical communication witha hemodialysis machine, an electrical output means that is in electricalcommunication with the bioimpedance analysis measurement unit and thatis attachable to a body segment, the electrical output means is adaptedto apply electrical current to the body segment, an electrical inputmeans that is in electrical communication with the bioimpedance analysismeasurement unit and is attachable to a body segment, the electricalinput means being adapted to receive the current transmitted through thebody segment and transmit the same to the bioimpedance analysismeasurement unit. The bioimpedance analysis measurement unit is adaptedto determine body segment resistivity based on the current transmittedthrough the body segment and the bioimpedance analysis measurement unitprovides feedback to the hemodialysis machine in response to the bodysegment resistivity. In one preferred embodiment, the device includesmeans for applying pressure to the body segment, the pressure applyingmeans is in electrical communication with the bioimpedance analysismeasurement unit. Optionally, the pressure applying means includes apressure cuff that is adapted to encircle the body segment. Preferably,the electrical output means includes at least two injector electrodes,the electrical input means includes at least two measurement electrodes.The injector electrodes and the measurement electrodes are secured tothe pressure cuff. Optionally, the pressure cuff includes at least oneconductive band with opposing ends and a conductive plate positionedadjacent one of the ends of the conductive band, the conductive bandextends substantially the length of the pressure cuff. The conductiveplate is arranged to electrically contact the conductive band at a pointalong the length of the same wherein the distance between the conductiveplate and the point of contact of the conductive band is substantiallyequal to the circumference of the body segment, and wherein thebioimpedance analysis measurement unit is adapted to electricallydetermine body segment circumference based on the distance between theend of the band adjacent to the plate and the point of contact of theplate along the length of the band.

One embodiment is a device for monitoring hydration status in ahemodialysis patient comprising a bioimpedance analysis measurementunit, an electrical output means, optionally comprising at least twoinjector electrodes, being in electrical communication with thebioimpedance analysis measurement unit and being attachable to a bodysegment, the electrical output means being adapted to apply electricalcurrent to the body segment, an electrical input means, optionallycomprising at least two measurement electrodes, being in electricalcommunication with the bioimpedance analysis measurement unit and beingattachable to a body segment, the electrical input means being adaptedto receive the current transmitted through the body segment and transmitthe same to the bioimpedance analysis measurement unit. The bioimpedanceanalysis measurement unit is adapted to determine body segmentresistivity based on the current transmitted through the body segment.Optionally the device includes a hemodialysis machine. Optionally thedevice includes means for applying pressure to the body segment,optionally a pressure cuff. The pressure applying means being inelectrical communication with the bioimpedance analysis measurementunit.

One embodiment of the device includes a pressure cuff with at least oneconductive band with opposing ends and a conductive plate positionedadjacent one of the ends of the conductive band. The conductive bandextends substantially the length of the pressure cuff and is arranged toelectrically contact the conductive band at a point along the length ofthe same wherein the distance between the conductive plate and the pointof contact of the conductive band is substantially equal to thecircumference of the body segment, and wherein the bioimpedancemeasurement unit is adapted to electrically determine body segmentcircumference based on the distance between the end of the band adjacentto the plate and the point of contact of the plate along the length ofthe band.

The present invention includes a device for calculating cardiac outputthrough bioimpedance measurements of a patient comprising a bioimpedancemeasurement unit, a first electrical output means being in electricalcommunication with the bioimpedance analysis measurement unit and beingattachable to an arm segment, the first electrical output means beingadapted to apply electrical current to the arm segment, a secondelectrical output means being in electrical communication with thebioimpedance analysis measurement unit and being attachable to a legsegment, the second electrical output means being adapted to applyelectrical current to the leg segment, a first electrical input meansbeing in electrical communication with the bioimpedance analysismeasurement unit and being attachable to an arm segment, the electricalinput means being adapted to receive the current transmitted through thearm segment and transmit the same to the bioimpedance analysismeasurement unit, a second electrical input means being in electricalcommunication with the bioimpedance analysis measurement unit and beingattachable to a leg segment, the electrical input means being adapted toreceive the current transmitted through the leg segment and transmit thesame to the bioimpedance analysis measurement unit, a first pressureapplying means for applying a maximum pressure to the arm segment, thefirst pressure applying means being in electrical communication with thebioimpedance analysis measurement unit, a second pressure applying meansfor applying a maximum pressure to the arm segment, the second pressureapplying means being in electrical communication with the bioimpedanceanalysis measurement unit, means for selectively electronicallyconnecting the bioimpedance analysis measurement between the firstelectrical input and output mans and the second electrical input andoutput means, and wherein the bioimpedance analysis measurement unit isadapted to selectively measure stroke volume in the arm and leg segmentsby bioimpedance analysis.

Other objects, features and advantages of the invention will be readilyapparent from the following detailed description of a preferredembodiment thereof taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1A and 1B each represent a stylized 3-dimensional view of a bodysegment, that illustrates the principle of measuring resistivityaccording to one embodiment of the present invention. FIG. 1A representsthe situation in which no external pressure is applied to the segmentand the blood vessels are uncompressed. FIG. 1B illustrates thesituation in which external pressure is applied to the segment and theblood vessels are compressed.

FIG. 2 is a block diagram of a measurement system according to thepresent invention.

FIG. 3 is a graph of relative changes in systolic blood pressure,ultrafiltration rate, resistivity of different body segments, andrelative blood volume over time during hemodialysis.

FIG. 4 is a graph of post-dialysis resistivity compared to dry weight inten male hemodialysis patients.

FIG. 5 is a graph showing the relationship between blood pressure andresistivity of a limb segment in ten male hemodialysis patients at theend of hemodialysis.

FIG. 6 is a graph of Body Mass Index versus resistivity in a limbsegment in ten male healthy subjects.

FIG. 7 is a graph showing the change is body segment impedance inrelation to the change in body segment blood volume due to arterialpulses.

FIG. 8 is a graph showing changes in impedance of a limb segment inrelationship to pressure cuff pressure.

FIG. 9 is a bar graph showing the correlation between a series of tenmale hemodialysis patients' post dialysis limb segment resistivity, withand without correction for dry weight, and the limb segment resistivityin a series of ten healthy male subjects.

FIG. 10 is a diagram of a pressure cuff for measurement of thecircumference of a body segment and for use in measurement of segmentalbioimpedance when the body segment is compressed or uncompressed. Shownis a front view with the covering partially cut away, and in FIG. 10A, apartial back view showing the conductive plates.

FIG. 11 is a block diagram of a device according to the presentinvention that also provides a means for determining cardiac output.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method of determining hemodialysis andperitoneal dialysis patients' hydration status, or more specifically,dry weight, to facilitate the appropriate dialysis prescription. Theinvention comprises a means of determining and monitoring theresistivity of the patient's body or body segment, and hence the correctdry weight or desired hydration status of a patient undergoing dialysis.The invention further provides a method for determining and monitoringvarious physiologic parameters of the patient undergoing dialysis,including but not limited to heart rate (HR) and cardiac output (CO).

From a physiological point of view, in healthy people the amount offluid in the interstitial compartment should be a relatively constantvalue within a small range. Thus, this value should be the criterion toindicate the degree of a patient's body hydration.

We have found that the refilling volume of a peripheral body segment,such as an arm (upper extremity) or leg (lower extremity), is animportant indicator of a dialysis patient's hydration status or dry bodyweight. In one aspect, the present invention provides a means toseparately measure, by segmental bioimpedance analysis (SBIA), thedegree of regional body hydration, including fluid volume in theinterstitial compartment and the intravascular compartment, in order todetermine a patient's fluid status and dry body weight.

One preferred embodiment of the present invention comprises a means tomeasure the resistivity of a body segment. The body segment may be thewhole body, preferably a limb segment, more preferably a leg or armsegment, and most preferably a thigh segment. As shown in FIG. 1A, theresistivity of a body segment is measured by the placement ofmeasurement electrodes at points L1 and L2, separated by a distance L.One of skill in the art will appreciate that while distance L may vary,it is preferably about 10 cm. The resistivity between L1 and L2 isdenoted as R. Also shown in FIG. 1A is a cross-section of the bodysegment with the interstitial compartment denoted as T and blood vesselsdenoted as B. Optionally, a means to compress the body segment isprovided, for example a pressure cuff 3 that surrounds the body segment.When the body segment is not compressed, for example when the pressurecuff 3 is uninflated, the blood vessels are uncompressed and theresistivity R reflects the resistivity of both the interstitialcompartment T and the intravascular compartment B. As shown in FIG. 1B,when the body segment is compressed, for example by inflating thepressure cuff, to a pressure above about the systolic blood pressure,optionally up to about 240 mmHg, the blood vessels are compressed andsubstantially all of the blood volume contained within the intravascularcompartment of the body segment is forced out of the body segment. Whenthe resistivity between the electrodes placed at L1 and L2 is measuredunder such circumstances, the resistivity value ρ₁ represents theresistivity of the interstitial compartment of the body segment.

The principle of measurement of segmental bioimpedance provides a meansto measure segmental resistivity and may be explained with reference toFIGS. 1A and 1B. Segmental resistivity is calculated using the formula:

ρ_(measure) =Aρ ₁ /L(m·Ω)

Where ρ_(measure) is the measured segmental resistivity; A is thecross-sectional area of the segment (A=C²/4π, where C is thecircumference of the segment) When no pressure is applied to the bodysegment the cross sectional area A₀ represents the cross sectional areaof the body segment including that of the blood vessels, when pressureof at least systolic blood pressure is applied the cross sectional areaA_(p) is that of the body segment minus the cross sectional area of theblood vessels ; ρ is resistivity as measured by bioimpedance analysis;and L is the distance between the measurement points (i.e. the distancebetween measurement electrodes).

The measured resistivity of the body segment depends on a number offactors including the frequency of the injected current and the bodymass index (BMI). Preferably a single frequency, optionally multiplefrequencies (multi-frequencies) are used. Injected frequencies fromabout 1 kHz to about 1000 kHz, more preferably from about 1 kHz to about50 kHz, most preferably from about 1 kHz to about 10 kHz are utilized.BMI reflects fat content, and is defined as the body weight in kgdivided by the square of the height in meters (weight/height²) and istypically measured in kg/m². In order to distinguish betweenintravascular and interstitial fluid, preferably the body segment iscompressed, optionally by a pressure cuff, preferably a blood pressurecuff (BP cuff) to produce a pressure (P) sufficient to squeeze bloodvolume out of the studied segment over a few seconds. Thus, tworesistivity values can be measured: ρ₀ (uncompressed body segment, P=0mmHg) and ρ_(p) (body segment is compressed to a pressure from aboutsystolic blood pressure up to P_(max)=240 mmHg).

Based on the resistivity measurement, dry weight and the excess bodyfluid is calculated according to the equation:

Dry weight=pre-weight−V _(excess)

Where pre-weight is the weight of the patient at a time prior to thecompletion of dialysis, preferably prior to the initiation of dialysis,and V_(excess) is the excess volume of fluid in a patient's body thatmust be removed in order to achieve dry weight.

The equation to calculate V_(excess) is:

V _(excess)=(k ₁ /k ₂)·ρ_(cal) ·BMI _(p)  (Equation 1)

Where

BMI_(p) is the body mass index (kg/m²) of the dialysis patient,

ρ_(cal) is resistivity (m×Ω) which is obtained by the equation asfollows:

ρ_(cal) =λ·BMI _(p)+200, (m×Ω);  Eq. 1.1

 Where

λ is the fitting coefficient of resistance/density (Ω/(kg/m³)) derivedfrom a linear regression equation based on the relationship between BMIand resistivity in healthy subjects; and

where BMI_(p) is the BMI of the patient.

It is anticipated that specific coefficients will be derived forspecific populations for improved results. By way of example, which isnot intended to be limiting, utilizing experimental data shown in FIG. 6from the ten healthy male subjects set forth in Example 1 (below), λ=13(Ω/(kg/m³). The value of 200 (m·Ω) is the baseline of resistivity whenBMI has a minimum value (that must be larger than zero)

k₁/k₂ (m/ohm) is a coefficient constant depending on the measurementvalue according to body geometry and tissue resistivity of theindividual patient; and where

k ₁ =C ₁ ×C ₂

 and where C₁ is obtained by means of a linear regression equationderived from suitable experimental data correlating the relationshipbetween resistivity of a body segment and clinical dry weight values indialysis patients; and

C ₂=ρ_(cal)/(ρ_(cal)−ρ_(measure))

 where ρ_(measure) is the resistivity in a body segment of a dialysispatient measured by SBIA while the body segment is compressed to apressure of at least about systolic pressure;

Again, it is anticipated that specific coefficients will be derived forspecific populations for improved results, but by way of example, whichis not intended to be limiting, utilizing experimental data shown inFIG. 4 and derived from the ten male hemodialysis patients described inExample 1, C₁=2.2×10⁻¹ kg/(m·Ω) and C₂=3.5

K₂ is the average value for BMI in a population of healthy subjects. Ina sample of ten healthy males described in Example 1, the average BMI,K₂ is approximately 26.8 kg/M², and

ρ_(cal) is the calculated resistivity (m·Ω)=(13×BMI_(p))+200 (Ω·m³/kg)

The measurement system comprises a high speed, low noise, acquisitionand multi-frequency bioimpedance measurement unit, such as is known toone of ordinary skill in the art, preferably a Xitron 4200s (XitronTechnologies, San Diego, Calif.). Connected to the bioimpedancemeasurement unit, the system includes an electrical output meansattachable to a body segment, the electrical output means preferablycomprising at least two injector electrodes for application to a bodysegment and for the injection of current into the body segment. Thesystem can apply a single frequency of current, or optionally multiplefrequencies of electricity (multi-frequencies) ranging from about 1 kHzto about 1000 kHz, more preferably from about 1 kHz to about 50 kHz,most preferably a single frequency from about 1 kHz to about 10 kHzthrough the injector electrodes. The system further comprises anelectrical input means that is adapted to receive the electrical currenttransmitted from the output means and through the body segment and tothen transmit the current to the bioimpedance analysis measurement unit.The input means comprises at least two measurement electrodes forapplication to the body segment for the receiving and transmission, tothe BIA measurement unit, of current transmitted through the selectedsegment. The electrodes may be made of Ag/AgCl film, conductive rubber,or other appropriate materials which are readily apparent to one ofordinary skill in the art. The injector and measurement electrodes areconnected electrically to the BIA measurement unit. This electricalconnection may be accomplished by a number of means readily apparent toa person of ordinary skill in the art, but preferably by electricalcables.

In one preferred embodiment of the present invention, the electrodes areincorporated into a pressure cuff suitable for surrounding andcompressing the body segment. A single cable optionally may incorporateboth the electrical wires to the injector and measurement electrodes andthe air tubing connected to the pressure cuff. Such a cable is used toconnect the pressure cuff to the measuring unit and an optional airpump. Alternatively, separate electrical cables and a separate air hosemay be employed. Optionally, the pressure cuff incorporates a means forelectrically measuring the circumference of the body segment. An exampleof a preferred pressure cuff configuration 3, which is not intended tobe limiting in any way is disclosed in FIG. 10. The pressure cuff 3 is ablood-pressure cuff type device that comprises a substantiallyrectangular form suitable for wrapping around a body segment, such thatthe body segment is encircled by the device. The pressure cuff iscomposed of a fabric or other flexible material that preferably iscapable of being easily cleaned and/or decontaminated. Material that issuitable will be readily apparent to one of ordinary skill in the art.The pressure cuff also includes a means for securing the device on thebody segment, such as a Velcro® system or other such securing system 26,as will be readily apparent to one of ordinary skill in the art.Contained within the pressure cuff 3 is a flexible air-bladder 25 orsimilar means of compressing the body segment, and applyingsubstantially circumferential pressure of at least about systolic bloodpressure to the body segment. The air-bladder is connected to an airhose through which air can be moved to inflate or deflate theair-bladder. The pressure cuff preferably includes at least two injectorelectrodes 9 and at least two measurement electrodes 10 incorporatedtherein. The injector and measurement electrodes are electricallyconnected, preferably by electrical wires 20 and 21 respectively, to acable connector 27, or other means of electrically connecting thepressure cuff 3 to a bioimpedance measurement unit. At least one,preferably two conductive bands 24 extend substantially the length ofthe pressure cuff, such that the length of the bands is at least equalto the smallest normal body segment circumference. The bands arecomposed of a material of stable resistivity. Suitable material includesCu—Sc alloy or conductive rubber. Other suitable material will bereadily apparent to one of ordinary skill in the art. The pressure cuffalso. comprises at least one and preferably two conductive plates 28located at the end of the pressure cuff opposite to the end with thesecuring means 26. The conductive bands 24 and conductive plates 28 areelectrically isolated from one another and each is connected, preferablyby wires 22 and 23, respectively, to a means of measuring resistivity.The band(s) 24 and plate(s) 28 are arranged on the pressure cuff, suchthat when the pressure cuff is wrapped around the body segment, theplate(s) 28 electrically connects with the band(s) 24 at a location orlocations along the length of the belt such that the distance, measuredalong the length of the pressure cuff, from the plate(s) 28 to the pointof contact on the band(s) 24 is substantially equal to the circumferenceof the body segment. The circumference of the body segment then can bedetermined electrically according to the equation:

L _(b1) =R 1 ×A 1/ρ1

Where L_(b1) is the length of the band between the end of the pressurecuff 3 closest to the end where the plate(s) is (are) located and thelocation at which the plate 28 contacts the band 24;

where R1 is the resistivity of the band between its end closest to theend at which the plate(s) is (are) located and the location at which theplate 28 contacts the band;

where A1 is the cross-sectional area of the band;

and ρ1 is the resistivity of this material.

In this manner, by determining the resistivity of the length of theband(s) that substantially equals the circumference of the body segment,the circumference of the body segment can be determined electrically. Inthis embodiment, it is preferred that the pressure cuff be securelyapplied prior to each measurement in order to more accurately measurebody segment circumference.

Another embodiment comprises a device for controlling a hemodialysismachine. In this and in other embodiments disclosed herein, an exampleof a hemodialysis machine suitable for use in or with the invention isthat disclosed in U.S. Pat. No. 5,580,460 to Polaschegg. An example,which is not intended to be limiting in any way, is depicted in FIG. 2.In addition to the BIA measurement unit 1, the measurement system alsocomprises one or more of an air pump 2 to produce pressure to inflatethe pressure cuff 3, a control unit 4 to transfer signals from themicroprocessor in order to operate the pump, a microprocessor system 5which is at least a minimal computer with fast data transfer, rapidaccess and a memory space sufficiently large to permit the manipulationand analysis of the inputted data, a means of communicating with thedialysis machine 6 whereby control signals are sent to and received fromthe dialysis machine allowing the control of ultrafiltration rate andother hemodialysis parameters according to body hydration status, adisplay 7 that shows the result of online measurement and an operationinterface 8 to input individual patients' parameters to monitor andcontrol dry weight and optionally a means of communication to a standardpersonal computer (PC) or other device. Optionally, data including, butnot limited to, resistance, resistivity, cuff pressure and heart rate istransmitted to the PC by a RS 232 interface or another standardinterface in ASCII or other format such that the waveforms ofresistivity, pressure values, heart rates and other parameters can beobserved, stored, or manipulated on the PC. The block diagram in FIG. 2shows injector electrodes 9 and measurement electrodes 10, optionallyincorporated into the pressure cuff 3. The injector and measurementelectrodes are attached, preferably by electrical wiring 11, to the tothe output sockets I_(a) and I_(b) and input (measurement) sockets V_(a)and V_(b) of the BIA measurement unit 1, and the air pump 2 is connectedto the pressure cuff by an air hose 12.

In this embodiment, various patient specific parameters are input intothe microprocessor system 5 by means of the operation interface 8.Inputted data and other data optionally are displayed in the display 7.The microprocessor system 5 is connected to the BIA measurement unit 1by a means of transmitting signals to the BIA measurement unit andsignaling the BIA measurement unit to send electrical current to theinjector electrodes 9. When such an electrical current is sent throughthe injector electrodes into the body segment, the current is detectedby the measurement electrodes and transmitted back to the BIAmeasurement unit for processing, the derived date being transmitted tothe microprocessor system. The microprocessor system is also connectedto the pump control unit 4 which is capable of sending signals to theair pump 2 to inflate and deflate the pressure cuff 3, allowingbioimpedance measurements to be made with the pressure cuff inflatedand/or deflated. The microprocessor system is also connected to thehemodialysis machine by a communication means 6, whereby signals can besent to the hemodialysis machine permitting changes in the hemodialysisprocedure, such that the patient's hydration status may be altered.

In one embodiment of the present invention, the ultrafiltration rate isvaried by the microprocessor in response to on-line monitoring of thepatient's segmental resistivity in order to achieve the patient's properdry weight or other desired hydration status, and to prevent hypotensionduring hemodialysis. Optionally, the individual ultrafiltration rate isvaried using a time course function related to the slope of changes insegmental resistivity (explained below) during dialysis to optimize thehemodialysis treatment.

The present invention provides a means to determine hemodialysis andperitoneal dialysis patients' dry weight to facilitate the appropriatedialysis prescription. In one preferred embodiment of the presentinvention segmental bioimpedance is continuously measured in a bodysegment during hemodialysis. The body segment may be any portion of thebody or the entire body, but is preferably a limb segment, morepreferably a leg or arm segment, most preferably a thigh segment. Therelative changes in the value of resistivity is calculated from aboutevery 20 minutes to about every one minute, more preferably about every10 minutes, even more preferably about every 5 minutes, and mostpreferably about every minute. The circumference of a body segment,preferably a thigh segment or optionally an arm segment, is measured,preferably at the start, optionally at the end of treatment, andpreferably intermittently during dialysis, more preferably from aboutevery 10 minutes to about every 20 minutes, in order to derive thecross-sectional area of the segment. Preferably, at least two injectorelectrodes and at least two measurement electrodes are attached to thebody segment. The electrodes may optionally be incorporated within apressure cuff 3 in the manner set forth above (see, for example, FIG.10).applied with a pressure cuff and may more preferably be applied aspart of a pressure cuff-electrode combination device.

Periodically, current is injected into the body segment through injectorelectrodes and the current transmitted through the body segment isreceived by the measurement electrodes. Current from the measurementelectrodes then is transmitted to the BIA measurement unit, whichdetermines the resistance of the body segment and optionally transmitsthe calculated resistance to a microprocessor system that calculates theresistivity according to the method disclosed herein, and which, inturn, may control a hemodialysis machine. Multiple resistivity datapoints are obtained over time, a curve is derived, and the slope of thecurve determined. The slope of the curve approaching zero indicates thata substantially constant resistivity has been achieved, therebyreflecting that dry weight has been substantially attained. As theresistivity curve slope approaches zero, the hydration status of thepatient approaches dry weight. Optionally, ultrafiltration may beprolonged or otherwise modified until dry weight is achieved during theongoing hemodialysis treatment session or hemodialysis may be prolongedduring the next hemodialysis treatment to remove the excess fluid andachieve dry weight.

In another embodiment, suitable for both hemodialysis and peritonealdialysis patients, comparison of the body segment resistivity,preferably post dialysis resistivity, of dialysis patients to the bodysegment resistivity of healthy subjects is used to determine thepatients' hydration status and optionally the appropriate end point fordialysis. The circumference of a body segment is measured, preferably atthe start and optionally at the end of treatment. The body segment maybe the whole body, a limb segment such as a leg, arm, or otherextremity, and is preferably a thigh segment or an arm segment.Preferably, at least two injector electrodes 9, at least two measurementelectrodes 10, and optionally a pressure cuff 3 are attached to the bodysegment (see, for example, FIG. 10). Preferably, the electrodes areincorporated within to the pressure cuff. At least once, preferablyabout the time that the dialysis treatment is completed, bioimpedance ofthe body segment is measured. Optionally bioimpedance is measured at thestart and end of the dialysis treatment, periodically, during most orall of the dialysis treatment, optionally from about every 10 minutes toabout every 20 minutes. Bioimpedance is measured optionally with thebody segment uncompressed or preferably, with the body segmentcompressed, preferably by inflation of the pressure cuff. The injectionand measurement of current is coordinated to correspond with time pointswhen the pressure cuff is substantially fully inflated or substantiallydeflated.

To measure resistivity, current is injected into the body segmentthrough injector electrodes and the current transmitted through the bodysegment is received by the measurement electrodes and transmitted to theBIA measurement unit for calculation of the resistivity of the bodysegment, the derived data optionally being transmitted to themicroprocessor system, which, in turn, according to the method disclosedherein.

To obtain a range of normal resistivity values, the bioimpedance ofhealthy subjects is measured repeatedly at specific body segments, whichmay be the whole body, preferably a limb segment, more preferably a legor an arm segment, most preferably a thigh segment, over about 15 minuteperiods. From these values, a set of normal resistivity values isderived that correlates with dry weights. Preferably a large group ofhealthy subjects is studied to produce a set of normal resistivityvalues for a specific population. Optionally, determination ofresistivity in subsets of the healthy population can be performed inorder to more precisely correlate resistivity values with dialysispatient's dry weight. For example, because fat mass is often animportant factor affecting the measurement of body fluid volumes bybioimpedance analysis, mainly due to the association between theconductivity of skin or fat free mass and the amount of fat,stratification of bioimpedance values according to BMI, gender or ageoptionally may be undertaken.

At any particular time point, the resistivity of the dialysis patient'sbody segment is compared to the resistivity of the equivalent bodysegment in healthy subjects, in order to determine the patient'shydration status. When the resistivity of the dialysis patient's bodysegment is substantially equal to the resistivity in normal subjects,the dialysis patient is determined to be substantially at dry weight,and preferably the patient's body weight is measured. Subsequently, thepatient's body weight measured at a different time point can be comparedto the body weight measured at the time that the patient was at aboutdry weight in order to determine ΔW, the difference between thepatient's actual weight and dry weight, and thereby the patient's stateof hydration. Using ΔW, the patient's dialysis protocol may be modifiedso that dry weight is achieved post-dialysis. By way of example, whichis not intended to be limiting, if a patient is determined, by comparingresistivity values to those of healthy subjects, to be at dry weight ata mass of X kg, and if at the time of the next dialysis treatment thepatient's mass is Y kg and Y>X, then ΔW=Y−X, reflecting the amount ofexcess fluid to be removed by dialysis to achieve dry weight.Preferably, repeated determinations of dry weight by bioimpedanceanalysis are performed periodically to provide greater precision indetermining the dry weight of a particular patient.

It is known that the bioimpedance of a body segment changes as the bloodpumped by the heart enters and leaves the body segment with each heartbeat. By frequent or continuous injection of current and measurement ofsegmental bioimpedance, a wave form that reflects the pulse can bederived. Based on this information, the present invention provides ameans to determine and monitor the heart rate of a patient prior to,during, or after hemodialysis by means of BIA, according to theequation:

HR=60/(T _(i+1) −T _(i))

where HR is the heart rate in beats per minute; and T_(i+1)−T_(i) is thetime period between peaks of any two successive heart beat inducedimpedance waves, T_(i) and T_(i+1), as shown in FIG. 7.

In another embodiment of the invention, BIA is optionally used todetermine cardiac output in individuals, including, but not limited tohealthy subjects, and dialysis patients prior to, during, or followingdialysis. Estimation of CO is based on the assumption that there is ahigh degree of symmetry in the distribution of blood vessels on bothsides of the body and the fact that total blood volume per pulse (strokevolume) can be measured in the segments of the arm (SV_(arm)) and leg(SV_(leg)) using bioimpedance simultaneously (preferably measuring thestroke volume from an arm and an ipsalateral leg (i.e., on the same sideof the body)).

The equation used to calculate cardiac output is:

CO=2×HR(k ₃ ×SV _(arm) +k ₄ ×SV _(leg))(L/min)

where SV_(arm) and SV_(leg) are the stroke volume in the arm and in theleg respectively;

SV_(arm) and SV_(leg) are calculated using the following formulas:

SV _(arm) =ΔV _(A) /N _(A) and SV _(leg) =ΔV _(L) /N _(L)

 where ΔV_(A) is the change in blood volume in the arm and ΔV_(L) is thechange in the blood volume in the leg between the time point of maximalcuff pressure (shown as segment point A in FIG. 8, during which timesubstantially all the blood volume is squeezed from the limb segment)and the time point when the pressure cuff is deflated (Shown as point Bin FIG. 8, during which time blood volume is refilled by the strokevolume). N_(A) and N₁ are the number of pulses during changes inimpedance from peak point (A) to baseline (B) respectively.

The values for ΔV_(A) and ΔV_(L) are calculated as follows:

ΔV _(A)=−ρ_(b) L ² ΔZ _(A) /Z _(A) ² and ΔV _(L) =−ρ _(b) L ² ΔZ _(L) /Z_(L) ²  Equation 2

where ρ_(b) is the resistivity of blood, L is the length of the body orlimb segment between the electrodes, and Z_(A) and Z_(L) are eachrespective impedance values. Calculations of ΔV_(A) and ΔV_(L) areperformed according to the method of J. G. Webster in, MedicalInstrumention Application and Design, 3^(rd) Ed., Wiley, New York, 1998pp. 357-362, which is hereby incorporated herein by reference, in itsentirety. The coefficients k₃ and k₄ are coefficients of calibration forindividuals in ΔV_(A) and ΔV_(L) respectively. The calibration isperformed by injecting from about 5 ml to about 150 ml into a veindistal to the arm segment in which resistivity is to be measured, whilethe resistivity is measured continuously in the arm segment. As the waveof increased volume (ΔV) passes through the segment, there is a changein resistance (ΔR) in relation to the volume injected. Using therelationship between ΔV/ΔR, k₃ and k₄ are calibrated.

The calibrating process provides the information about how a change inresistance per ohm is related to a known change in volume (ΔV/ΔR). Bydefinition, define k_(c)=ΔV/ΔR as a calibration coefficient, where ΔV isthe volume of injected saline (ml) and ΔR is the change in resistance inthe calibrating segment. Thus, k₃ is defined by equation as follows:

k ₃ =k _(c) ×ΔZ _(A)/(N _(A) ×V _(A))

Where ΔZ_(A) is the change in impedance in the arm, V_(A) is volumecalculated by set, and N_(A) is number of pulses. Similarly, theequation k₄=k_(c)×ΔZ_(L)/(N_(L)×V_(L)) is used to calibrate for changesin the volume of a leg.

One embodiment of a system such as that disclosed in FIG. 2, butadditionally being capable of measuring cardiac output is shown in FIG.11. Included are two sets of electrodes 9 and 10 and 9′ and 10′,preferably incorporated into two pressure cuffs 3 and 3′ adapted to beattached to a leg segment (not shown) and to an ipsalateral arm segment(not shown), both sets of electrodes being connected to a digitalswitch, via wiring 11 and 11′, capable of rapidly switching between eachset of electrodes, so that measurements may be taken from either the legsegment or the arm segment substantially simultaneously. Preferably thedigital switch 30 has the capacity to achieve a sampling frequency of atleast about 200 Hz and, more preferably, greater than 1 kHz. Optionally,there is a means to send a control signal from a computer 31 to thedigital switch so that the sample frequency can be changed as needed.

Listed below are a series of examples of the present invention. Theexamples contained herein are intended to illustrate, but are notintended to limit the scope of the invention.

EXAMPLE 1

Twenty healthy subjects (Table 1) and thirteen hemodialysis patients(Table 2) were studied, the latter during hemodialysis. Shown in Tables1 and 2 are their mean ages, weights and body mass indices (BMI). Dataare presented as mean value ±SD

TABLE 1 Healthy subjects Age Weight BMI n (years) (kg) (kg/m²⁾ Male 1040.8 ± 5 83.1 ± 21.6 27.1 ± 5.0 Female 10   35 + 9 64.3 ± 9.7  24.2 ±3.2

TABLE 2 Hemodialysis patients Age Dry Weight BMI n (year) (kg) (kg/m²⁾Male 10 48.5 + 12.8 76.8 ± 16.4 26.8 ± 4.3 Female 3 65 + 14 60.5 ± 16  23.7 ± 3.5

EXAMPLE 2

Segmental bioimpedance was measured continuously every 10 minutes duringhemodialysis using 6 electrodes all on the left side of the body. Twoelectrodes, one on the hand and one on the foot, were used to injectcurrent. Measurement electrodes were placed on the wrist, shoulder, hipand ankle. Resistivity was measured in the wrist-shoulder segment(Varm), the shoulder-hip segment (Vtrunk), and in the ankle-hip segment(Vleg). Also measured were systolic blood pressure (SBP), relative bloodvolume or hematocrit (RBV), and the ultrafiltration rate (UFR). In thisway, blood volume and segmental extracellular volume (ECV) in the leg,arm and trunk were calculated. The results are shown in FIG. 3. TheX-axis is time in minutes, the Y-axis the relative change in value withthe value of a particular parameter at the start of hemodialysis beingequal to 100%. After continuing ultrafiltration changes in ECV of theleg became small that the slope was nearly horizontal i.e. approached 0,which indicates that little fluid was available for ultrafiltration anddry weight had been achieved. Comparing the curves of ECV trunk, leg andarm, it can be seen that the leg is the preferred body segment for dryweight analysis.

EXAMPLE 3

The resistance and resistivity of a limb segment with and withoutinflation of a pressure cuff was measured at the start and at thecompletion of a hemodialysis treatment and ΔW was calculated. Table 3shows that in a series of patients ΔW was ≠0 at the end of hemodialysis,indicating that these patients were not at their clinical dry weight atthe end of hemodialysis.

TABLE 3 Results in ten male patients and ten male healthy subjects AreaR₀ R_(p) ρ₀ ρ_(p) ΔW Subjects (cm²) (Ω) (Ω) (Ωcm) (Ωcm) (L) Start HD86.7 ± 20 41.1 ± 6.7 44.3 ± 8  354.8 ± 95    383 ± 106 4.31 ± 1.3  EndHD  81 ± 19 53.1 ± 8.4 56.6 ± 9  428.6 ± 116 457.6 ± 125 0.17 ± 0.41Healthy 98.2 ± 26 49.6 ± 10  54.5 ± 11 500.3 ± 60   547 ± 60 0

In confirmation (FIG. 3) both ρ₀ and ρ_(p) at the end of HD were loweror higher in patients than in healthy subjects. This indicates that mostpatients were overhydrated (i.e., excess fluids were not removed) whilesome were dehydrated by the treatment and had lower than normal fluidvolume. It is a purpose of the present invention to have all patients insubstantially the same range as healthy subjects after treatment (noteresistivity is inversely related to interstitial volume). FIG. 4 shows ahigh correlation between ΔW and ρ_(p) which indicates that ρ_(p) iscapable of being used for analysis of patients' segmental hydration sothat dry weight could be predicted by this technique.

EXAMPLE 4

FIG. 5 shows the clinical correlation of resistivity and blood pressureat the end of hemodialysis in 10 male patients. FIG. 4 shows theclinical correlation between resistivity and body hydration at the endof hemodialysis in these patients. The normal range for resistivity isshown as a solid line. The results demonstrate that the hemodialysispatients were not at the correct dry body weight as indicated by thistechnique at the end of the treatment. Most were over-hydrated, howeverthe three denoted by the symbol ∘ were underhydrated. Using Equation 1the individual dry weight was calculated according to the healthysubjects' ρ₀ and ρ_(p) and BMI. The patients' dry weight aftercorrection compared to healthy subjects and uncorrected dry weight areshown in FIG. 9.

Throughout this application, various articles and patents arereferenced. Disclosures of all of these publications are herebyincorporated herein by reference in their entireties. The presentinvention may be embodied in other forms without departing from thespirit or essential attributes thereof and accordingly reference shouldbe made to the claims rather than to the foregoing specification asindicating the scope thereof

We claim:
 1. A method for determining the hydration status of a dialysispatient comprising the steps of: measuring the resistivity of a bodysegment of the patient; correlating the measured resistivity withpredetermined normal dry weight values; and deriving the patient'shydration status.
 2. The method of claim 1, wherein the resistivity ofthe interstitial fluid in the body segment is measured.
 3. The method ofclaim 1, wherein the resistivity of the body segment is determined whileapplying a pressure of at least about systolic blood pressure.
 4. Themethod of claim 3, wherein the pressure is applied from about 120 mmHgto about 240 mmHg.
 5. The method of claim 1, wherein the body segment isa limb segment.
 6. The method of claim 1, wherein the body segment is athigh segment.
 7. A method for determining a hemodialysis patient's dryweight comprising the steps of: periodically measuring the resistivityof a body segment during hemodialysis; comparing successive resistivitymeasurements; and identifying the patient's dry weight when asubstantially constant resistivity is reached.
 8. The method of claim 7,wherein the resistivity of the body segment is measured from about every5 minutes to about every 20 minutes during hemodialysis.
 9. The methodof claim 8, wherein the resistivity of the body segment is measuredabout every 10 minutes during hemodialysis.
 10. The method of claim 7,wherein the resistivity of the body segment is measured at a pressure ofat least about systolic blood pressure.
 11. The method of claim 10,wherein the pressure is from about 120 mmHg to about 240 mmHg.
 12. Amethod for dialysing a patient to the patient's dry weight comprisingthe steps of: measuring the resistivity of a body segment of thepatient; correlating the measured resistivity with predetermined normaldry weight values; deriving the patient's hydration; and continuinghemodialysis until the resistivity of the body segment correlates withthe predetermined normal dry weight values.
 13. The method of claim 12,wherein the resistivity of the body segment is measured at a pressure ofat least about systolic blood pressure.
 14. A method for hemodialysing apatient to the patient's dry weight comprising the steps of:periodically measuring the resistivity of a body segment duringhemodialysis; comparing successive resistivity measurements; anddiscontinuing hemodialysis when a substantially constant resistivity isreflected.
 15. The method of claim 14, wherein the resistivity of thebody segment is measured at a pressure of at least about systolic bloodpressure.
 16. The method of claim 14, wherein the resistivity of thebody segment is measured from about every 5 minutes to about every 20minutes during hemodialysis.
 17. A method of monitoring the heart rateof a hemodialysis patient comprising the steps of: determining a timeinterval between two successive bioimpedance wave peaks; and multiplyingthe reciprocal of the time interval by 60 to obtain the heart rate. 18.A method of calculating the cardiac output of a patient in need thereofcomprising the steps of: measuring the stroke volume in an arm segmentby bioimpedance analysis; substantially simultaneously measuring thestroke volume in an ipsalateral leg segment by bioimpedance analysis;summing the stroke volume in the arm segment and the stroke volume inthe leg segment; and multiplying the sum by twice the heart rate toobtain the cardiac output.
 19. The method of claim 18, wherein strokevolume of the arm segment is calculated by applying an external maximumpressure to the arm segment and determining the change in blood volumein the arm segment between the point of maximum pressure and the pointat which no external pressure is applied divided by the number of heartbeats between the two points in time, and wherein the stroke volume ofthe leg is calculated by applying an external maximum pressure to theleg segment and determining the change in blood volume in the legsegment between the point of maximum pressure and the point at which noexternal pressure is applied divided by the number of heart beatsbetween two points in time.
 20. A device for controlling a hemodialysismachine comprising: a bioimpedance analysis measurement unit inelectrical communication with a hemodialysis machine; an electricaloutput means being in electrical communication with the bioimpedanceanalysis measurement unit and being attachable to a body segment, theelectrical output means being adapted to apply electrical current to thebody segment; an electrical input means being in electricalcommunication with the bioimpedance analysis measurement unit and beingattachable to a body segment, the electrical input means being adaptedto receive the current transmitted through the body segment and transmitthe same to the bioimpedance analysis measurement unit; and wherein thebioimpedance analysis measurement unit is adapted to determine bodysegment resistivity based on the current transmitted through the bodysegment and wherein the bioimpedance analysis measurement unit providesfeedback to the hemodialysis machine in response to the body segmentresistivity.
 21. The device of claim 20 further including means forapplying pressure to the body segment, the pressure applying means beingin electrical communication with the bioimpedance analysis measurementunit.
 22. The device of claim 21 wherein the pressure applying meansincludes a pressure cuff, the pressure cuff being adapted to encirclethe body segment.
 23. The device of claim 22 wherein the electricaloutput means includes at least two injector electrodes, the electricalinput means includes at least two measurement electrodes, and whereinthe injector electrodes and the measurement electrodes are secured tothe pressure cuff.
 24. The device of claim 23 wherein the pressure cufffurther includes at least one conductive band with opposing ends and atleast one conductive plate positioned adjacent one of the ends of theconductive band, the conductive band extends substantially the length ofthe pressure cuff, the conductive plate is arranged to electricallycontact the conductive band at a point along the length of the samewherein the distance between the conductive plate and the point ofcontact of the conductive band is substantially equal to thecircumference of the body segment, and wherein the bioimpedancemeasurement unit is adapted to electrically determine body segmentcircumference based on the distance between the end of the band adjacentto the plate and the point of contact of the plate along the length ofthe band.
 25. A device for monitoring hydration status in a hemodialysispatient comprising: a bioimpedance analysis measurement unit; anelectrical output means being in electrical communication with thebioimpedance analysis measurement unit and being attachable to a bodysegment, the electrical output means being adapted to apply electricalcurrent to the body segment; an electrical input means being inelectrical communication with the bioimpedance analysis measurement unitand being attachable to a body segment, the electrical input means beingadapted to receive the current transmitted through the body segment andtransmit the same to the bioimpedance analysis measurement unit; and amicroprocessor system in electrical communication with the bioimpedanceanalysis measurement unit; wherein the bioimpedance analysis measurementunit is adapted to determine a body segment resistance based on thecurrent transmitted through the body segment and to transmit the bodysegment resistance to the microprocessor system, and wherein themicroprocessor system is adapted to calculate a body segmentresistivity, correlate the body segment resistivity with predeterminednormal dry weight values, and to derive the hydration status of thehemodialysis patient.
 26. The device of claim 25 further including meansfor applying pressure to the body segment, the pressure applying meansbeing in electrical communication with the bioimpedance analysismeasurement unit.
 27. The device of claim 26 wherein the pressureapplying means includes a pressure cuff, the pressure cuff being adaptedto encircle the body segment.
 28. A device for monitoring hydrationstatus in a hemodialysis patient comprising: a bioimpedance analysismeasurement unit; an electrical output means being in electricalcommunication with the bioimpedance analysis measurement unit and beingattachable to a body segment, the electrical output means being adaptedto apply electrical current to the body segment, wherein the electricaloutput means includes at least two injector electrodes; an electricalinput means being in electrical communication with the bioimpedanceanalysis measurement unit and being attachable to a body segment, theelectrical input means being adapted to receive the current transmittedthrough the body segment and transmit the same to the bioimpedanceanalysis measurement unit, wherein the electrical input means includesat least two measurement electrodes; and a means for applying pressureto the body segment, the pressure applying means being in electricalcommunication with the bioimpedance analysis measurement unit, whereinthe pressure applying means includes a pressure cuff, the pressure cuffbeing adapted to encircle the body segment; wherein the bioimpedanceanalysis measurement unit is adapted to determine a body segmentresistivity based on the current transmitted through the body segment,and wherein the injector electrodes and the measurement electrodes aresecured to the pressure cuff.
 29. The device of claim 28 wherein thepressure cuff further includes at least one conductive band withopposing ends and a conductive plate positioned adjacent one of the endsof the conductive band, the conductive band extends substantially thelength of the pressure cuff, the conductive plate is arranged toelectrically contact the conductive band at a point along the length ofthe same wherein the distance between the conductive plate and the pointof contact of the conductive band is substantially equal to thecircumference of the body segment, and wherein the bioimpedancemeasurement unit is adapted to electrically determine body segmentcircumference based on the distance between the end of the band adjacentto the plate and the point of contact of the plate along the length ofthe band.
 30. A hemodialysis machine including a device for monitoringhydration status in a hemodialysis patient, the device comprising: abioimpedance analysis measurement unit; an electrical output means beingin electrical communication with the bioimpedance analysis measurementunit and being attachable to a body segment, the electrical output meansbeing adapted to apply electrical current to the body segment; and anelectrical input means being in electrical communication with thebioimpedance analysis measurement unit and being attachable to a bodysegment, the electrical input means being adapted to receive the currenttransmitted through the body segment and transmit the same to thebioimpedance analysis measurement unit; wherein the bioimpedanceanalysis measurement unit is adapted to determine body segmentresistivity based on the current transmitted through the body segment.31. A device for calculating cardiac output through bioimpedancemeasurements of a patient comprising: a bioimpedance measurement unit; afirst electrical output means being in electrical communication with thebioimpedance analysis measurement unit and being attachable to an armsegment, the first electrical output means being adapted to applyelectrical current to the arm segment; a second electrical output meansbeing in electrical communication with the bioimpedance analysismeasurement unit and being attachable to a leg segment, the secondelectrical output means being adapted to apply electrical current to theleg segment; a first electrical input means being in electricalcommunication with the bioimpedance analysis measurement unit and beingattachable to an arm segment, the electrical input means being adaptedto receive the current transmitted through the arm segment and transmitthe same to the bioimpedance analysis measurement unit; a secondelectrical input means being in electrical communication with thebioimpedance analysis measurement unit and being attachable to a legsegment, the electrical input means being adapted to receive the currenttransmitted through the leg segment and transmit the same to thebioimpedance analysis measurement unit; a first pressure applying meansfor applying a maximum pressure to the arm segment, the first pressureapplying means being in electrical communication with the bioimpedanceanalysis measurement unit; a second pressure applying means for applyinga maximum pressure to the arm segment, the second pressure applyingmeans being in electrical communication with the bioimpedance analysismeasurement unit; and a means for selectively electronically connectingthe bioimpedance analysis measurement unit between the first electricalinput and output means and the second electrical input and output means;wherein the bioimpedance analysis measurement unit is adapted toselectively measure stroke volume in the arm and leg segments bybioimpedance analysis.